High-frequency wattmeter



Feb. 14, 1950 T. MORENO HIGH-FREQUENCY WATTMETER 2 Sheets-Sheet 1 FiledJune 1'7, 1944 INVENTOR THEODORE MORENO Patented Feb. 14, 195oHIGH-FREQUENCY WATTIKETER Theodore Moreno, Garden City, N. Y., assignerto The Sperry Corporation, a corporation of Dela- Application June 17,1944, serial' No. 540,821

2 Claims.

This invention relates particularly to wattmeters and has reference moreparticularly to a novel calorimeter type of ultra high frequencywattmeter suitable for measuring the power in concentric lines and waveguides.

The novel wattmeter of this invention serves to dissipate theelectromagnetic energy of the concentric line or waveguide directly inthe coolant employed, thereby not only simplifying the structure of thewattmeter but also obtaining high accuracy in adapting the wattmeter foruse in the high power ranges as well as for low powers.

The principal object of the present invention is to provide a novel highfrequency wattmeter employing a coolant as the dielectric of aconcentric line or Wave guide, as the case may be.

Another object of the present invention is to provide a novel wattmeterthat has very broad band response, being operablersubstantiallyindependent of frequency of the supplied energy over a wide band.

Still another object of the present invention is to provide a novelwattmcter of the above character especially valuable for use in thehigher power ranges, said wattmeter being extremely accurate and ofunvarying characteristics.

A further object of the present invention is to provide a novelwattmeter wherein the energyabsorbing medium is accurately matched tothe supply line through the intermediary" of a line section employing adiierent suitable dielectric material such as polystyrene. This linesection performs the dual function of matching the liquid-filled sectionto the supply line and sealing the liquid in the line.

Other objects and advantages will become apparent from thespecification, taken in connection with the accompanying drawingswherein the invention is embodied in concrete form.

In the drawings,

Fig. l is a perspective view, partly broken away, of one form ofwattmeter of the present invention adapted for use in connection with aconcentric line; t

Fig. 2 is enlarged central sectional view of Y the structure of Fig. 1:

Fig. 3 is a sectional view of a somewhat modiiied construction;

Fig. 3a is a fragmentary portion of the structure of Fig. 3 showing asomewhat modiiled construction; and

Fig. 4 illustrates the wattmeter of the present invention as applied toa waveguide.

Similar characters of reference are used in all 2 of the above figuresto. indicate corresponding parts.

Referring now to the structure illustrated in Figs. l and 2 of thedrawings of the novel wattmeter of this invention, an outer conductor Iis shown mounted within a housing 2, the said conductor l having aninner conductor portion 3 surrounded by a liquid dielectric 4, which maybe water for example, and which dielectric serves directly to absorb theelectromagnetic energy travelling down the concentric line. The increasein heat of the dielectric 4, which when taken with the rate of flow is ameasure of the power, is indicated by calorimetric methods.

`In the drawing the dielectric is shown circulating within theconcentric line section surrounding the inner conductor portion 3. ',I'o.facilitate this circulation, tubes 5 and 6 are attached to the outerconductor, one at each end of the liquid dielectric section, and theliquid dielectric is forced through these tubes via branch tubes 5' and6', thereby maintaining a steady ilow of dielectric in surroundingrelation to inner conductor 3 and within the outer conductor. I.

In order to measure the amount of heat ab-` sorbed by the dielectric, i.e. to indicate the power of the line, a suitable thermocouplearrangement may be employed, using thermocouples 1 and 8 mounted in thetubes 6 and 5 respectively. the said thermocouples being connected tothe meter 9 directly or through a suitable amplifier if desired. Meansfor measuring the rate of ow of the liquid is preferably employed toobtain uniform results. The meter 9 may be calibrated directly in powerunits if desired. Instead of using the thermocouple arrangement shown,it is possible to employ thermometers I0 and Il, as y shown in Fig. 3,mounted in the tubes 5 and 6 respectively and by subtracting thereadings of the thermometers and measuring the rate of liquid ilow, theenergy absorbed may be measured.

In order that the wattmeter present a matched load to the line, thelength of liquid-filled line should be such as to provide suiicientattenuation so that the reflected wave of energy from the shorted end ofthe line is small enough to be neglected. Ihe impedance of theliquid-lled line is then transformed to that of the air-filled line bythe use of an intermediate section, which employs a solid dielectricsuch as polystyrene to provide a water seal. The dimensions of thissection of line employing a low-loss solid dielectric are chosen so thatit will perform the required electrical transformation. For example, thetransformation will be effected when the inter mediate section of lineis electrically a quarterwavelength long and has a characteristicimpedance that is the geometric mean of the characteristic impedances ofthe adioining air-filled and liquid-lled line sections.

This is an approximation that neglects any imaginary component in thecharacteristic impedances of these adjoining line sections. but theseimaginary components, if they are ap'- preciable, may be taken into.account by more precise calculations. Y

The following is a typical approximate calculation, such as is performedin designing a wattmeter of this type.

The loss per unit length in the water-filled line is given by a=2"1.3 umy cia/unit length wherein Assuming that er' is equal to 75, and thetangent of i' equal to .13,

20 db. are ample for a termination, so ten inches of water line would besuitable. The polystyrene section must be electrically a quarterwavelength, i. e. M4 long.

The impedance of the polystyrene section must be such as to match theair line section to the water line section.

Zo=impedance of the air-filled line. Zo/\/75=impedance of thewater-iilled line. Za'=impedance of the polystyrene line. Zo=Zo/751/4The form of wattmeter so far described is somewhat limited in bandwidth, it being noted that a deiinite length of transformer section isrequired for a given wavelength. In order to make this wattmeter presenta matched impedance over a wide band of frequencies, it is necessary tochoose the diameters of the polystyrene and water-lled sections so as tomaintain a constant impedance along the line (neglecting the phase anglein the characteristic impedance of the water section). For the impedancein the dielectriciilled line section to be equal to that of theairiilled line, the radius of the inner conductor a' in the dielectricfilled region should be d an (5)# where a is the inner radius of theair-fllled line,

b is the outer radius of both lines, and

e is the dielectric constant of the dielectric medium separating theconductors.

For example, if 2b=.805, 2a=.375. pou.=2.52

a' in the polystyrene dielectric region:

'water-:75 a' in the water dielectric region:

Such a constant impedance wattmeter is shown in Fig. 3. In this iigure,the outer conductor is designated I6 and the small inner conductor ofthe water-filled line is designated I1, while Il is the polystyrenesection, which may be quite short, and not necessarily aquarter-wavelength in length. 'I'he inner conductor of the air-filledline is designated I9 and that of the polystyrene section is designated20.

Also, if desired, the small central conductor of the liquid dielectricline section can be reinforced by suitable means. such as a polystyrenecoating shown at 2i in Fig. 3a and the diameters adjusted tov maintain aconstant impedance.

It will be noted that with water as the power absorbing liquid, thecentral conductor becomes extremely small, i. e., 00053" radius, if itis desired to have the wattmeter matched at all wavelengths for whichthere is suiiicient attenuation in the water-filled line. However, itshould be noted that it is not necessary to use water as the absorbingliquid. Some other liquid could be used having a lower dielectricconstant. Also, if the diameter of such water filled line is reduced apractical'amount to, say, .050", and the polystyrene section designed tomatch such a line to the air-filled line, the band width over which thewattmeter is matched will be considerably increased over that shown inFigs. l and 2.

The novel wattmeter of this invention is not designed for use inconnection with concentric lines alone, but the same may also be used inconnection with waveguides. The structure shown in Fig. 4 illustratesthe wattmeter applied to a waveguide. In this structure the waveguide 22is shown having a water-filled section 22' through which the water iscirculated as shown in connection with Figs. 1 and 2. In order to matchthe impedance of the air-filled line with that of section 22', a metalinsert 23 is employed together with a polystyrene insert 24, theseinserts being electrically a quarter-wavelength in length. It will'beborne in mind that the wattmeter will remain matched as long as there issufiicient attenuation in the water-filled section, so that reflectionsfrom the end of the line will be negligible, and this attenuation perunit length would be linearly proportional to the frequency if thedielectric constant and power factor of water remained unchanged withchanging frequency.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A microwave wattmeter comprising a coaxial transmission line having aliquid filled section, temperature responsive means for measuring powerabsorbed by said liquidfilled section, said liquid serving as adielectric power absorbing medium a supply line section. and a matchingsection between saidv supply line section and said liquid iilledsection, said matching section including inner and outer conductorswhose diameters bear a different ratio to each other than the inner andouter conductors of said liquid iilled section, and a body oi' solid.dielectric material lling the space between said conductors in saidmatching section, said body acting as a seal to maintain said liquid insaid liquid lled section.

2. A microwave wattmeter comprising a coaxial transmission line having aliquid lled section, temperature responsive means for measuring powerabsorbed by said liquid illled section, said liquid serving as adielectric power absorbing medium a supply line section, and a matchingsection between said supply line section and said liquid lled section,said matching'section including an inner conductor whose diameter is'substantially greater than that of the inner conductor of said liquidlled section, and a body of solid dielectric material lling the spacebetween said conductors in said matching section,

6 said body acting as a seal to maintain said liquid in said liquid lledsection.

THEODORE MORENO.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS

